1. Field of the Invention
The present invention relates to a color cathode ray tube apparatus, and particularly, to a color cathode ray tube apparatus of a dynamic focus type which corrects a deflection aberration caused by a magnetic field generated by a deflection yoke.
2. Description of the Related Art
In general cases, a color cathode ray tube apparatus has an envelope consisting of a panel 1 and a funnel 2 integrally connected with this panel 1. A fluorescent screen 3 is formed on the stripe-like or dot-like three-color fluorescent material layers which irradiate blue, green, and red light rays, and a shadow mask 4 provided with a number of apertures is attached inside the screen 3, such that the mask faces the screen 3. On the other hand, an electron gun which emits three electron beams 6B, 6G, and 6R is provided in the neck 5 of the funnel 2. In addition, a deflection yoke 8 for generating horizontal and vertical deflection magnetic fields is provided outside the funnel 2. Further, the three electron beams 6B, 6G, and 6R are deflected by the horizontal and vertical deflection magnetic field in the direction toward the fluorescent screen 3 through the shadow mask 4. The fluorescent screen 3 is scanned by electron beams 6B, 6G, and 6R to display a color image.
This kind of color cathode ray tube apparatus particularly uses an electron gun 7 as an in-line type electron gun which emits three electron beams 6B, 6G, and 6R arranged in one line and extending on one same vertical plane. Meanwhile, an in-line type color cathode ray tube apparatus using a self-convergence method in which three electron beams 6B, 6G, and 6R arranged in line are subjected to self-concentration has been widely practiced, with a horizontal deflection magnetic field of a pin-cushion type and a vertical deflection magnetic field of a barrel type being generated.
Conventionally, this kind of electron gun 7 comprises a cathode which controls electron emission therefrom and focuses emitted electrons to form three electron beams 6B, 6G, and 6R, an electron beam generator section consisting of a plurality of electrodes arranged in order next to the cathode, and main electron lens section consisting of a plurality of electrodes which focuses the three electron beams 6B, 6G, and 6R obtained from the electron beam generator section onto a fluorescent screen 3.
In the color cathode ray tube apparatus stated above, it is necessary to appropriately focus three electron beams 6B, 6G, and 6R emitted from the electron gun 7, in order to attain good image characteristics on the fluorescent screen 3. However, three electron beams 6B, 6G, and 6R are subjected to astigmatic aberration in case where a horizontal deflection magnetic field of a pin-cushion type and a vertical deflection magnetic field of a barrel type are used as non-uniform magnetic fields which deflect three electron beams 6B, 6G, and 6R emitted from the electron gun 7, like in a color cathode ray tube apparatus of an in-line type using the self-convergence method. To explain this case with respect to the vertical deflection magnetic field of a pin-cushion type, for example, electron beams 6 (6B, 6G, and 6R) are influenced under forces in the arrow directions 11H and 11V applied by the vertical deflection magnetic field 10, as is shown in FIG. 2A, and the beam spot 12 on a peripheral portion of the fluorescent screen is influenced by deflection aberration and is remarkably deformed. The deflection aberration which influences the electron beams is caused since electron beams are excessively focused in the vertical direction, and a large halo (or bleeding) 13 appears in the vertical direction. The deflection aberration which influences the electron beams becomes larger as the tube has a larger size and as the deflection angle becomes larger, thereby remarkably deteriorating the resolution at the periphery of the fluorescent screen.
Japanese Patent Application KOKAI publication No. 61-99249 (corresponding to U.S. Pat. No. 4,814,670) discloses an electron gun which solves the problem of deterioration in resolution due to deflection aberration. These publications disclose electron guns having a basic structure illustrated in FIG. 3A. Specifically, each of those electron guns has first to fifth grids G1 to G5, and comprises an electron beam generator section GE, 4-pole element lens Q1, and a final focusing lens EL which are formed along the extending direction of the electron beams. The 4-pole lens QL of each electron gun is formed by in such a manner in which three non-circular electron beam through-holes 14a, 14b, and 14c as well as 15a, 15b, and 15c are formed in each of opposing surfaces of third and fourth grids G3 and G4.
The correction of deflection aberration incurred by the electron gun is expressed in an equivalent value using an optical lens, as shown in FIG. 4. Specifically, an electron gun is an electron gun of a dynamic focus method in which a 4-pole lens QL and a final focus lens EL are formed in order in the direction from the cathode K toward the fluorescent screen. In case of this electron gun, potentials at the third and fourth grids are substantially equalized to each other during non-deflection in which electron beams 6 from the cathode K land on the center of the fluorescent screen 3 so that the 4-pole lens QL does not substantially operate, while the electron beams 6 are appropriately focused on the center of the fluorescent screen 3 by the final focus lens EL, as indicated by continuous lines in the figure. On the contrary, the potential of the fourth grid is raised to form a 4-pole lens QL during deflection, so that the beams are diverged in the vertical direction and are focused in the horizontal direction. Simultaneously, the focusing effects are weakened in the vertical and horizontal directions. As a result of this, the electron beams 6 are insufficiently focused in the vertical direction, while the beams are influenced by the deflection aberration, i.e., focusing effects due to astigmatic aberration so that the beams are appropriately focused. Meanwhile, the total focusing in the horizontal direction are not substantially changed due to focusing effects of the 4-pole lens QL and reductions in focusing effects of the final focus lens EL, so that focusing is slightly insufficient. However, since the peripheral portion of the fluorescent screen 3 which the electron beams 6 reach is more distant from the electron gun than the center portion, the beams are appropriately focused in the vertical direction.
Thus, a color cathode ray tube apparatus which deflects three electron beams arranged in line and generated from the electron gun by means of non-uniform magnetic fields generated by the deflection yoke is influenced by astigmatic aberration due to the non-uniform magnetic fields, so that the beam spot in the peripheral portion of the fluorescent screen is deformed. The deflection aberration which influences the electron beams increases as the tube has a larger size and as the deflection angle becomes large, so that the resolution at the peripheral portion of the fluorescent screen is remarkably deteriorated. As an electron gun which solves the problem of deterioration in resolution, there has been a proposal of an electron gun of a dynamic focus method comprising electrodes consisting of first to fifth grids, as well as, an electron beam generator section, a 4-pole lens, and a final focusing lens which are formed in order in the direction from the cathode toward the fluorescent screen.
However, in this kind of electron gun using a dynamic focus method, since the third and fourth grids must be supplied with two kinds of middle voltages of 5 to 10 kV, there is a problem of withstanding voltages in a voltage supply section. In addition, since the connection lines for applying respective electrodes with predetermined voltages must be long, there are problems, e.g., the withstanding voltages in the tube are decreased and the focusing characteristics of the electron beams are deteriorated due to discharges and leakage currents.
In this respect, Japanese Patent Application KOKAI Publication No. 1-232643 (corresponding to U.S. Pat. No. 4,945,284) discloses an electron gun in which two adjacent grids among a plurality of grids are connected by a resistor device provided in the tube, with one of these two grid which is adjacent to the final acceleration electrode being applied with a dynamic voltage, and the other being applied with the dynamic voltage through a resistor device, thereby to attain only one kind of middle voltage supplied from outside the tube.
However, the resistance value of this resistor device is conventionally about 200 k.OMEGA., and sufficient consideration has not been taken with respect to setting of this resistance value. Specifically, when a disclosed resistance value of 200 k.OMEGA. is used, a potential difference does not occur between two electrodes connected by a resistor device and a multi-pole lens which will correct astigmatic aberration of the deflection magnetic field is not formed, even if a dynamic focus voltage is applied. Therefore, there is a problem that correction of astigmatic aberration is difficult.